Conventionally, a friction stir welding apparatus has been known in which upper and lower rotary tools inserted from a front surface side and a back surface side of a welding part of a metal plate are used to weld the metal plate by friction stirring with the upper and lower rotary tools (see JP 4838385 B2). In the friction stir welding apparatus, the upper and lower rotary tools have a cylindrical tool body, and a probe attached to the front end of the tool body. A shoulder part is formed around an attachment portion of the probe at the front end of the tool body. The friction stir welding apparatus performs the friction stir welding in a state of imparting a predetermined gap between the front ends of the probes of the upper and lower rotary tools.
Further, a friction stir welding apparatus has been known in which first and second rotary tools disposed on the front surface side and the back surface side of the welding part of the metal plate to face each other are used to weld the metal plate by friction stirring with the first and second rotary tools (see, for example, JP 4838389 B1). In the friction stir welding apparatus, one of the first and second rotary tools has a tool body having a shoulder part formed at a front end thereof and a probe (protruding portion) formed to protrude from the tool body, and the other thereof has a tool body having a shoulder part formed at a front end thereof and a recessed portion which stores the front end of the probe. The friction stir welding apparatus performs the friction stir welding in a state in which the protruding portion of one rotary tool is inserted into the recessed portion of the other rotary tool.